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Abstract: The decay of the rare nuclide 184Os by alpha emission to 180W has been theoretically predicted, but was previously never observed in experiments. Variable excesses of 180W were recently observed for iron meteorites, but the contribution to these excesses by 184Os-decay was regarded as insignificant. Here, we present combined 180W and Os–W concentration data for meteorites and terrestrial rocks, now indicating that the 180W heterogeneities can be explained by α-decay of 184Os. A combined 184Os–180W isochron for iron meteorites and chondrites yields a decay constant value of of (half life yr), in good agreement with theoretical estimates. The 184Os–180W decay system may constitute a viable tracer and chronometer for important geological processes like core formation, silicate differentiation or late accretion processes. This is illustrated by a measured 180W-deficit in terrestrial basalts relative to chondrites by parts in 10 000, consistent with core formation ∼4.5 Ga ago. [Copyright &y& Elsevier]

The δ 18 O value of seawater is presently buffered by high-temperature hydrothermal alteration and low-temperature weathering of lithospheric rocks. It is much debated whether the δ 18 O of seawater has been buffered to this steady-state value throughout Earth's history or, alternatively, whether it gradually increased towards the present value since the Archean. A third possibility is that the δ 18 O of seawater was buffered at a higher value before continents emerged, and has been buffered at its current value (δ 18 O ∼ −1‰) since the emergence of continents. In this contribution, we reconstruct the δ 18 O of Archean seawater from triple oxygen isotope (δ 17 O, δ 18 O) variations in >2.7 Ga ultramafic rocks that reacted with fluids that, in turn, had been derived from seawater in the Archean. The samples that were studied are peridotites from the Ivnartivaq complex, an ultramafic lens in the Archean Kuummiut terrane (Rae craton, southeast Greenland) that have unusually low δ 18 O values for peridotites (in olivine, 1.7 ‰ ≤ δ 18 O ≤ 4.6 ‰ , relative to VSMOW). Bulk rock trace element concentrations, mineral compositions and U-Pb dating of zircon grains in the peridotites indicate that these samples are metamorphic peridotites, that formed by the dehydration of serpentinites ∼2.7 Ga ago. The serpentinite protoliths, in turn, had formed by alteration of ultramafic cumulate rocks at high temperatures (250–450 °C) by fluids that were derived from seawater. Triple oxygen isotope variations in olivine from the peridotites indicate that the fluids could not have been derived from seawater with a significantly higher or lower δ 18 O value than that of seawater in the Phanerozoic, but could have been derived from seawater with a value of δ 18 O ∼ −1‰, i.e., the modern (ice-free) seawater composition. We conclude that the δ 18 O of seawater had reached its current steady state value by 2.7 Ga ago. • Archean peridotites from southeast Greenland contain olivine with 1.7‰ ≤ δ 18 O ≤ 4.6‰. • They are crystal cumulates that were altered by seawater to low- δ 18 O serpentinites. • The serpentinites became dehydrated during metamorphism ∼2.7 Ga ago. • We reconstruct the δ 18 O of >2.7 Ga seawater from triple O isotope variations in olivine. • Seawater had reached its current steady state value (δ 18 O ∼ −1‰) by 2.7 Ga ago. [ABSTRACT FROM AUTHOR]

Abstract: Heavy, proton-rich stable isotopes belong to the least abundant isotopes in the solar system. Their formation mechanisms and their stellar sources are most likely different from those of neutron-capture generated r- and s-process nuclei that comprise the majority of nuclides heavier than iron. Heavy p-nuclide abundances in meteorites are therefore potentially useful in deciphering distinct stellar contributions to the nascent solar system. We therefore conducted the first high-precision measurements of the heavy p-process isotope 180W, achieving a typical precision of ±0.7 ε-units for ca. 300ngW. Measured samples comprise metals from magmatic- and non-magmatic iron meteorites, as well as metal from one H4 chondrite (NWA 926) and two reduced terrestrial basalts (from Disko Island, Greenland and the Dzheltul''ski massif from Eastern-Siberia, Russia). The analyzed iron meteorites show clearly resolvable 180W anomalies of up to +6 ε-units. Conversely, the chondritic metal and both terrestrial samples exhibit 180W abundances indistinguishable from the standard value. As cosmogenic effects during space exposure of the meteoroids may have affected the 180W budget, cosmic-ray exposure of the meteorites has to be critically evaluated. We therefore propose a method to approximate cosmogenic contributions to the 180W signatures in order to unravel nucleosynthetic 180W abundance anomalies. Our study reveals significant cosmogenic effects only for the longest exposed meteorites, shifting 180W anomalies always to lower values (average cosmic-ray correction-factors can be estimated to lie between 0.01 and 0.30 ε 180W-units per 100Myr of exposure). Cosmogenic effects for most of the analyzed meteorites therefore appear to be negligible with respect to the analytical precision achieved for iron meteorites. In addition to cosmic-ray exposure, radiogenic effects can be caused by putative decay of 184Os or by decay of 180Ta in its ground state. Whereas potential alpha decay of 184Os could shift 180W anomalies to higher values (but only up to levels that are within the analytical error of ∼0.5 ε-units for most samples), no significant production of 180W could have occurred from 180Ta decay. Notably, we identified significant and systematic abundance variations in 180W between different iron meteorite groups, indicating that these isotope anomalies are characteristic for their entire parent asteroids. Our finding of decreasing excesses in 180W from early formed magmatic iron meteorites (+3.8±1.2 ɛ-units) towards later formed non-magmatic iron meteorites (+0.6±0.5 ɛ-units), the analyzed chondrite and both terrestrial rocks (−0.3±0.7 ε-units) may thus mirror progressive homogenization of 180W in the early solar nebula. This overall trend is also supported by a co-variation between 180W and metal segregation ages for the different iron meteorite groups as well as by a co-variation between 180W deviations and the respective asteroidal accretion ages. Such an interpretation would suggest progressive homogenization of the solar nebula within about ∼2.5 to ∼6Myr. [Copyright &y& Elsevier]

Abstract The stepwise acid digestion of primitive chondritic meteorites allows the identification of nucleosynthetic isotope anomalies that are otherwise hidden on the bulk rock scale. Here, we present combined Hf and W isotope data for acid leachates, residues, and bulk rock aliquots of several primitive chondrites that include highly precise analyses of the heavy p-process isotopes 174Hf and 180W. Including data for these two p-process isotopes enables, for the first time, the clear-cut discrimination between s- and r-process contributions to the Hf and W isotope inventory. Our analyses reveal Hf and W isotopic homogeneity at the bulk rock scale, but significant Hf and W isotope anomalies that are complementary between acid leachates and residues. Since both r- to p-process isotope ratios are invariant in leachates and residues, the observed anomalies can unambiguously be tied to variable contributions of carrier phases enriched in s-process nuclides, as previously inferred for, i.e., Mo and Ru in leaching experiments. Hafnium and W isotope anomalies co-vary in leachate and residue fractions from CM chondrites, whereas CO and CV chondrites are characterized by distinctly larger Hf isotope anomalies compared to W. This observation is most likely explained by more efficient homogenization of s-process W carrier(s) or, alternatively, by local redistribution of anomalous W into secondary less resistant phases during parent body and/or nebular processing. This implies the presence of different s-nuclide carrier phases for Hf and W. Several carriers of s-process-material appear to have been selectively dissolved by our leaching protocol, while contributions from r- and p-process Hf and W carrier phases appear invariant, possibly due to the generally more labile nature of their carrier phases during solar nebula and/or parent body processing. [ABSTRACT FROM AUTHOR]